1. Field of the Invention
The present invention relates: generally to respiratory systems supplying respiratory gas to users in hazardous atmospheres; more specifically to electric control means for the supply of respiratory gas to users in hazardous atmospheres; and most particularly to electric control means for the supply of respiratory gas to users in hazardous atmospheres by a respiratory system utilizing means for sensing partial pressure of a gas constituent.
2. General Background
Open circuit breathing systems utilize a compressed gas cylinder and a demand or continuous flow regulator to supply respiratory gas to the operator for inhalation. The exhaled gas is expelled to the ambient atmosphere.
Rebreather based breathing systems utilize the exhaled gas and recycle the unused oxygen contained in the operator's exhalation by means of a breathing loop. The carbon dioxide gas exhaled by the operator is removed by a chemical filter: a carbon dioxide scrubber. Rebreathers are classified into two major groups: semi-closed and fully closed.
Semi-Closed Rebreathers (SCR) are mechanical systems that expel a portion of the gas in the breathing loop at regular intervals. SCR systems are supplied by a premixed hyperoxic gas mixture or, less frequently, by a pure oxygen cylinder and a pure air cylinder. SCR systems are most often used in underwater environments as opposed to atmospheric, because use above land requires near normoxic respiratory gas and consequently high flow rates for the same.
Closed Circuit Rebreathers (CCR) are classified into either mechanical or electronically controlled. Mechanical systems rely on valves and actuating levers with the oldest and simplest closed circuit systems being pure oxygen. Electronic systems are able to maintain a desired level of oxygen in the breathing loop from a pure oxygen supply. In electronically controlled ‘mixed gas’ rebreathers the oxygen is added or ‘mixed’ with diluent gas such as air or helium. Whether mechanical or electronic a CCR system completely recycles all expired gas and replenishes the oxygen consumed by the operator. This replenishment is accomplished via purely mechanical means or via electronic and mechanical means. CCR systems provide maximum efficiency in oxygen usage and hence the longest operational time for a given cylinder volume.
Pure oxygen CCR systems are used in shallow water operations as well as land based operation, but are undesirable where:
a. pure oxygen could react with the environment,
b. long exposure times are required by the operator,
c. underwater depth exceeds 22 feet;
because of, respectively: the risk of rapid combustion, i.e. explosion; immediate and long term oxygen toxicity to human tissues at levels of oxygen above 0.5 atmospheres absolute (ATA); government regulations regarding oxygen toxicity.
Mechanically controlled mixed gas CCR systems are most typically controlled manually by the operator who is then responsible for maintaining the correct oxygen level at all times. These systems require the utmost attention by the operator to ensure that the oxygen level is correct and appropriate for the operator at all times. This requirement renders mechanically controlled manual CCR systems unsatisfactory for use by operators such as fire fighters and emergency first responders who are preoccupied by other continuously urgent tasks during operation.
Electronic or electrically controlled CCR systems monitor the oxygen levels in the breathing loop, via electrochemical oxygen fuel cells and an electronic controller, and maintain a desired oxygen level for the operator in the breathing loop by controlling a solenoid operated valve adding oxygen when open.
Mixed gas closed circuit rebreathers, mechanical or electronic, are supplied by two cylinders of compressed gas, one pure oxygen, the other pure air. The oxygen supply is used to replenish the oxygen consumed by the operator while the air supply is used to dilute the breathing mixture and to provide an emergency or bail out breathing gas supply.
Mixed gas closed circuit rebreathers are complicated and highly technical systems that require the operator to monitor feedback systems and critical processes for failure. The operator must have a high degree of training and use this type of device regularly in order to be proficient not only in the correct operation but also to be able to manage failure modes. Failure modes on mixed gas CCR systems are usually determined by information, or lack thereof, presented to the user on either primary and or secondary displays. In addition, the user must calibrate the system periodically for proper operation. Proper calibration, particularly, is critical to satisfactory operation of a mixed gas CCR system and requires very different tools and operating modes than those required in use of the system.
3. Discussion of the Prior Art
U.S. Pat. No. 5,050,939 issued to Clough discloses an underwater closed circuit mixed gas rebreather with three CPUs: the primary controls the solenoid; the secondary provides the display data and is a back up for solenoid control; while the third CPU is a data display back up that can indicate it is time to manually valve supply gas from a third cylinder that is a back up to the two cylinder main system. Back ups for the solenoid control and data display are provided, with three CPUs, and an emergency cylinder is added, but no provision is made for faulty sensors and the system relies wholly on redundancy in a cascading progression. A high level of technical training is required to operate the system.
U.S. Pat. No. 5,860,418 issued to Lundberg discloses an open circuit breathing system that measures variance of at least one ‘functional or status variable’ from a ‘control value’ with at least one sensor and a CPU whereby “the control circuit is activated . . . when there is a significant difference between these values.” (Col. 2, lines 10-15) As an alternative “the control circuit is activated manually, by pressing a start button, for instance.” (Col. 2, lines 21-23) No provision is made for faulty sensors and system redundancy is not suggested.
U.S. Pat. No. 6,003,513 issued to Readey et al. discloses a closed circuit rebreather with a partial pressure oxygen sensor in the counterlung used to control a valve from a cylinder with a CPU and a stepper motor. U.S. Pat. No. 6,302,106 issued to Lewis discloses a semi-closed circuit rebreather having both gas flows algorithmically controlled in accordance with depth to attain optimum partial oxygen pressure with the diminution of diluent gas at greater pressures being desired to avoid concentration in the blood stream. Neither the problem of faulty sensors nor the need for failsafe systems is addressed.
U.S. Pat. No. 6,712,071 issued to Parker discloses a mixed gas closed circuit rebreather with two independent sets of circuitry that ‘are interconnected in a primary and secondary relationship’: solenoid operation and display. Both sets of circuitry can perform either operation but the other must be switched manually in the event of power failure in the primary. A back up for the solenoid and the display is thus obtained. Parker is concerned with faulty partial pressure sensors and discloses use of three oxygen partial pressure sensors with rejection of a divergent value: “the signal from the sensor which differs from the each of the other two by the greatest amount is ignored” (col. 3, lines 58-60). Parker is also specific to oxygen levels greater than 0.50 for underwater use and requires a high level of technical training to operate.
Open, semi-closed circuit, and mechanical pure oxygen breathing systems are known dedicated to above land use in low oxygen, toxic, or otherwise hostile atmospheric conditions typically encountered in fire fighting and other emergency situations. U.S. Pat. No. 3,923,053 issued to Jansson utilizes a ‘unique scrubber apparatus’ suited to the semi-closed circuit rebreather utilizing two alternately filled, vented, and exhausted breathing bags to provide a gradual exhaustion of oxygen rather than the gradual increase obtained with a single cylinder of oxygen and a breathing bag characterizing previous semi-closed circuit breathers. There is no electronic controller and hence no sensing or monitoring capability.
U.S. Pat. No. 4,440,166 issued to Winkler et al. discloses a fail safe for power loss to the solenoid in an ‘Electrically and Mechanically Controllable Closed Circuit Respirator’ which has a spring loaded piston valve in a medium pressure chamber biased to open an alternative gas supply line upon loss of pressure owing to failure of the solenoid or the electrical system. ‘Oxygen sensing means’, in the breathing bag, and ‘electric control means’, for the solenoid, are specified without further detail. Switching to manual control, however, is indicated by a rise in pressure seen in a pneumatic pressure gauge and there is no suggestion of an electronic controller and it is presented as a pure oxygen rebreather only.
U.S. Pat. No. 4,640,277 issued to Meyer et al. discloses “a feedback mechanism responsive to facepiece pressure which actuates a supplemental second inlet air flow path to the facepiece during periods of high user demand” (Abstract) which utilizes a “novel expiration regulator system”: “an expiration valve spring controlled to hydraulically open to a first extent in direct response to positive face-piece pressures”, indicating a land use open circuit system, that further triggers a solenoid, or “electro assist mechanism which opens the expiration valve to a second and greater extent to reduce facepiece pressure”; or “a novel nonlinear spring mechanism” (col. 3, lines 1-31).
U.S. Pat. No. 5,036,841 issued to Hamilton discloses a “closed circuit breathing apparatus for supplying breathable air to a facepiece of a semi-closed circuit rebreathing system to be worn by a user while working in an irrespirable atmosphere” using a carbon dioxide scrubber, rebreather bag, and a “motorized fan . . . for continuously pumping air” (Abstract) which is ‘enriched’, i.e. slightly hyperoxic, with the oxygen above 20% and preferably about 30%.